This application is based on an application No. 11-71865 filed in Japan, the content of which is hereby incorporated by reference.
(1) Field of the Invention
The present invention relates to sealed batteries that are sealed by laser welding a closure cap onto an external battery casing. In particular, the invention relates to rectangular sealed batteries.
(2) Related Art
In recent years, sealed batteries have been widely used as a power source in portable electronic appliances such as mobile telephones, audio-video devices, and computers. Representative sealed batteries include alkaline batteries, such as nickel-hydrogen storage batteries and nickel cadmium storage batteries, and lithium-ion batteries.
Both cylindrical and rectangular sealed batteries are widely used, with rectangular batteries being subject to special attention due to their superior space-saving potential when used in portable devices.
Sealed batteries are constructed as follows. A cylindrical external casing with a closed bottom is first formed by subjecting a metal plate to a deep drawing process. A generator element formed of a positive and a negative electrode is placed inside the external casing and a closure cap is attached to an opening in the casing. The edges of the closure cap and the casing are then hermetically sealed to prevent the leakage of electrolyte or gas.
Nickel-plated steel or stainless steel is often used to make external casing, though the current trend is toward the use of an aluminum alloy, such as an alloy produced by adding manganese to aluminum, to reduce to weight of the cell.
Mechanical caulking is often used to form the seal. Since this technique is difficult for rectangular sealed batteries, such batteries are often sealed using laser welding. Laser welding is performed by having a laser beam scan around the edge of the closure cap and the edge of the opening in the external casing. The reliability of a cell and its life are greatly affected by how well this welded seal can be made.
When a battery is sealed using laser welding, it is desirable to suppress the power of the laser beam at the welded parts. This is because the closure cap is normally connected to one of the electrode terminals via a gasket, which is less likely to be damaged during the laser welding process if the power of the laser beam is restricted. The use of a low-power laser also makes the manufacturing process more energy-efficient.
When a laser source with the same power rating is used, a lower power setting for the laser at the welding parts enables the scanning speed to be raised. Also, a welding device used to form welded seals on batteries will usually include only one laser source that is split into several beams using optical fibers to allow a plurality of batteries to be welded simultaneously. If the power setting for the laser is lowered, a higher number of batteries can be simultaneously welded without changing the output of the laser source, thereby raising the efficiency of the manufacturing process.
When the power setting of the laser used to seal the battery is lowered, however, there is the problem of cracks tending to appear along the lines welded by the laser. Parts of the battery incident to the laser melt to form molten pools of metal. As these pools cool, they are pulled by the thermal stresses that occur in the surrounding metal When the power of the laser beam is low, there are sudden drops in temperature in the molten pools, which cause large thermal stresses.
Cracking is especially common in the welded parts when the external battery casing and closure cap are formed from an aluminum alloy sheet. This is because aluminum alloy has a lower tensile strength than iron or stainless steel, and because the thermal conductivity of aluminum is high, meaning that the molten parts cool rapidly.
As one example, Japanese Laid-Open Patent Application No. S61-3664 teaches a technique for producing a closure cap 131 with a turned-up outer edge 132 that is laser welded to the edge of the opening in the external casing 10 (see FIG. 8). In this technique, there is no linear path for heat to flow from the molten pools to the center of the closure cap, which reduces the dissipation of heat from the molten pools and so reduces the thermal stresses that occur at the welded parts.
When the outer edge of the closure cap is turned up, however, the width of the upper surface of the battery (equivalent to the length in the horizontal plane in FIG. 8) is increased by twice the width of the turned-up outer edge 132, represented by twice the thickness of the closure cap 131 given as W2 in FIG. 8. When the width of the entire cell is of the order of several millimeters, as in a slim rectangular sealed battery, however, this makes this technique very difficult to use.
It is an object of the present invention to provide a technique that facilitates the manufacturing of slim sealed batteries by suppressing the incident power of a laser beam used to form a welded seal while preventing cracks from forming.
The present invention achieves the stated object by forming a drop in the outer surface of a sealing plate that is laser welded to an opening in the external casing of a sealed battery. The presence of the drop means that the outer surface is higher at the perimeter of the sealing plate than in a more central position.
When the perimeter of the sealing plate is laser welded to the rim of the opening, the drop in the sealing plate cuts off the linear thermal conduction path from the welded parts to the center of the sealing plate, thereby suppressing the dissipation of heat from the welded parts. This reduces the thermal stresses that occur at the welded parts, and means that there is less cracking, even when the power of the laser beam is reduced.
One method of forming a drop in the outer surface of the sealing plate is to form a channel in the surface of a material used to form the sealing plate. Another method is to apply pressure to all but the perimeter parts of the material forming the sealing plate. A sealing plate can be easily formed in this way by forging, for example.
The drop can be formed in the sealing plate very close to the outside edge. This means that the sealing plate can be made narrower than for the case where the outside edges of the sealing plate are turned up, as shown in FIG. 8. As a result, the present invention provides a slim, rectangular sealed battery that is narrower than conventional batteries, but does not suffer from cracking in the welded seal.
Cracking is especially a problem when the external casing and sealing plate are formed from an aluminum alloy. This means that the suppression of cracks by the present invention is especially noticeable for batteries formed of this material.